The present invention relates to a coated type of magnetic recording media for high-density recording such as video tapes or magnetic discs.
So far, magnetic recording media of the coated type using an acicular form of ferromagnetic powders such as .gamma.-Fe.sub.2 O.sub.3 have been used for audio tapes, video tapes and magnetic discs. These magnetic recording media of the coated type make longitudinal (or planar) recording with the magnetic layer that the easy axes of .gamma.-Fe2O3 particles are oriented in the longitudinal direction. However, some limitation is placed on high-density recording achieved by longitudinal magnetization, because self-demagnetization increases with increases in recording density.
With such situations in mind, there has been proposed the so-called perpendicular recording mode in which magnetic recording is made by perpendicular magnetization, using a combination of a medium including a magnetic layer of perpendicularly oriented magnetic powders with a single-pole head. The perpendicular recording mode makes it possible to achieve some considerable improvement in recording density, because the internal demagnetizing field of the magnetic layer has the property of approaching zero with increases in recording density. For magnetic powders for the coated type of perpendicular recording medium, hexagonal ferrite powders in hexagonal plate geometry, such as barium ferrite powders, have been used, because they are readily oriented perpendicularly. A problem with the perpendicular recording medium is that the medium or a single-pole head usually used in conjunction with it wears away due to sliding. However, when a magnetic head for the longitudinal magnetic recording, for instance, an MIG type magnetic head is used together with the perpendicular recording medium, we have other problems, the asymmetry of readback pulse, getting the overwriting properties for lack of sufficiency of the magnetic head, and so on. This is particularly true of digital recording.
It is thus considered that even higher recording density will actually be achieved along an extension of the conventional longitudinal magnetic recording mode. For instance, a recording mode that takes the advantage of a component oriented perpendicularly with respect to the magnetic layer without recourse to the longitudinal magnetic recording mode--in which the easy axes are oriented within the surface of the magnetic layer in the recording track direction--is envisaged. An example of this mode has already been proposed in JP-A-3-280215 that discloses a coated type medium using ferromagnetic hexagonal ferrite powders, in which residual magnetization conforms to the relation of longitudinal&gt;perpendicular&gt;widthwise, and the (squareness) ratio of residual magnetization to saturation magnetization is at least 0.6 in the longitudinal direction, 0.3 to 0.7 inclusive in the perpendicular direction, and at most 0.3 in the widthwise direction. It is noted, however, that the magnetic recording medium set forth in this publication has for its object to keep the balance between long and short wavelength outputs in the video-related recording art.
Hexagonal ferrite powders are well fit for perpendicular recording, and are now under research as to their application to longitudinal recording media making use of their sharp magnetization inversion. These powders are used in the above-mentioned publication as well. Unlike ordinary acicular magnetic powders, hexagonal ferrite powders in hexagonal plate geometry are so increased in interactions between the magnetic particles that the actual degree of orientation of the particles found under a transmission electron microscope (TEM), etc., is worse than the orientation of the particles expected from the magnetization curve (B-H loop). To put it another way, the microscopic arrangement of the particles is not in agreement with the macroscopic magnetic properties of the particles. Parameters showing the orientation of magnetic powders in a magnetic layer include the above-mentioned squareness ratio and degree of orientation (that is given by Squareness Ratio in the orientation direction/Squareness Ratio in the direction perpendicular to the orientation direction). In the case of the hexagonal ferrite powders, however, even when the squareness ratios and degrees of orientation, both in the recording track direction and in the direction vertical to the magnetic layer, assume good-enough values, satisfactory read/write characteristics are not obtained in actual recording and reproduction, unless the particles are actually in alignment. In particular, there is some considerable degradation of S/N.
The present invention has been made in view of such situations, and has for its one object to provide an longitudinal magnetic recording medium having a perpendicularly oriented component, which is reduced in terms of the asymmetry of readback pulse, improved in terms of recording density and output, and excellent in S/N, and for its another object to provide a process enabling such a magnetic recording medium to be fabricated easily.